wtf is a ca180?
you probably won't pick up a 2L 180SX for 6k, but a 1.8 auto should be pretty easy, there's a fair few on the market too, esp with the P plater turbo restriction laws coming in.
it'll set you up nicely to move onto the torana, as you'll have practice in a RWD car.
also keep in mind that if you're only gonna keep the car for a year and a half, and you buy a 3k civic and mod it for another 3k, by the time you sell it, you'll probably only get 4k back. However a 6k 180sx will resell for about 6k. You won't lose as much on a stock car. You'll make no money on modifications.